1. Field of the Invention
The present invention generally relates to battery charging devices and methods, and more particularly to a device and method for charging a plurality of batteries.
Recently, many electronic devices, particularly, portable electronic devices have been designed to be equipped with a plurality of battery packs. Further, a lithium-based secondary battery or cell having a high energy density has widely become used in order to increase the battery usable or operable time. An example of such a lithium-based secondary battery is a lithium-ion cell. In the near future, another lithium-based battery having a higher energy density, such as a lithium-polymer battery, will be used as a secondary battery in practice.
The present invention relates to a device and method for charging such a secondary battery having a high energy density.
2. Description of the Related Art.
Conventionally, a battery charging device is available which can charge a plurality of lithium-based secondary batteries. In a conventional serial manner, the secondary batteries are subjected to charging one by one until the selected battery is completely charged up. In a conventional parallel manner, the batteries are connected in parallel and are simultaneously subjected to charging. If it is required to reduce the charging time, the parallel charge will be selected. If it is required to replace a battery with another battery, the serial charge will be selected. The serial charge can simply be implemented, whereas it takes a comparatively long charging time. The parallel charge requires a comparatively short charging time, whereas it is not suitable for a usage such that a battery is replaced by another one.
Another battery charging device has been proposed in which either the serial charge or parallel charge is selectively carried out on the basis of the capacities of the batteries to be subjected to charge (see Japanese Laid-Open Patent Application No. 4-251528). If the batteries have sufficiently been charged with electricity, the batteries are connected in series. If the batteries have not sufficiently been charged, namely, the batteries do not have sufficient capacities, the batteries are connected in parallel ad are subjected to quick charge.
Yet another battery charging device has been proposed in which a plurality of batteries are charged one by one and are then subjected to simultaneous charge (see Japanese Laid-Open Patent Application No. 6-303729). In the proposed charging device, a charging current which flows in one of the batteries is detected. If the charging current detected becomes equal to or lower than a given threshold level, the charging current is switched to another battery. When charging currents respectively flowing through the batteries become equal to or lower than the given threshold level, all the batteries are simultaneously supplied with the charging currents. The time during which the charging currents are being supplied to the respective batteries is controlled by referring to a timer. The switching between the batteries is based on only the information concerning the charging currents.
However, the conventional battery charging device as disclosed in Japanese Laid-Open Patent Application No. 4-251528 is not designed by taking into consideration the secondary batteries having a high energy density such as lithium-based secondary batteries. Thus, it takes a long time to charge the lithium-based batteries so that the batteries cannot efficiently be charged.
The conventional battery charging device as disclosed in Japanese Laid-Open Patent Application No. 6-303729 switches the battery which is being charged to another battery by referring to only the magnitude of the charging current. Hence, an error in measurement of the charging current will not correctly define the battery switching timing.